BRANCHIAL SENSE ORGANS OP ICHTHYOPSIDA. 181 



branches with the skin, and though figuring the actual fusion of the 

 vagus ganglia with the sensory thickening, does not ascribe to the skin 

 any part in the formation of the ganglia. 



Like Van Wijhe, I cannot find in the vagus outgrowth itself any 

 real segmentation in its earliest stages. The first outgrowth from the 

 neural crest (fig. 33) is a broad uninterrupted band stretching from 

 just behind the glossopharyngeal, which it almost joins, to a consider- 

 able distance backwards. 



Like other posterior roots, this outgrowth grows outwards and 

 downwards towards the portion of epiblast just above the second, third, 

 fourth, and fifth branchial clefts, which are now just forming (fig. 33). 

 Here the epiblast forms a longish sensory thickening, with which the 

 vagus fuses. 



Portions of the vagus pass on (fig. 34) behind the rudiments of each 

 of the above-mentioned clefts, and form, as in other cases, the post- 

 branchial nerves. 



At the point of fusion with the skin, cells are proliferated from the 

 epiblast to form the ganglia. 



Soon, as pointed out by Vau Wijhe, we get the ganglion of the first 

 vagus cleft separated from the rest of the mass and fused with an 

 isolated thickening above the second true branchial cleft. 



For the rest of the vagus there is usually only one ganglionic mass, 

 which, however, ventrally, and by its post-branchial branches, shows 

 a division into three portions. This mass lies over the last three 

 clefts, and is to be regarded as made up of the fused ganglia of the 

 three branchial sense organs of these clefts, with the addition, how- 

 ever, of rudiments of nerve elements of a certain number of clefts 

 which have disappeared, and even in the ontogeny hardly present 

 traces of their former existence. In Torpedo, however, as first noticed 

 by Wyman, 1 there is a rudiment of one cleft which never breaks 

 through to the surface, and which is therefore never functional. 2 The 

 rudiment of this cleft is very obvious in horizontal longitudinal 

 sections of certain stages, and is represented in fig. 47. Here there is 

 a considerable hypoblastic depression (cl. vi) of the pharynx just 

 behind the last or fifth branchial cleft. 



Corresponding to it is a shallower but still marked epiblastic 



1 Wyman, " Observations on the Development of Raja batis" 'Mem. Amer. Acad, of Arts 

 and Sciences, ' vol. ix, 1864. 



* This paper of Wyman's was not accessible, and the statement in the text is given from 

 Balfour's ' Embryology,' vol. ii, 



